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This haze is caused by airplanes, and it is gradually whitening blue skies, says Charles Long of NOAA’s Earth System Research Laboratory in Boulder, Colorado. “We might be actually conducting some unintentional geoengineering here,” Long said at a press conference this week at the American Geophysical Union Fall Meeting in San Francisco.

Geoengineering involves the manipulation of an environmental process in such a way, usually deliberate, that it affects the Earth’s climate. For instance, previous researchers have proposed combating global warming by intentionally seeding the atmosphere with small particles, or aerosols, to scatter some sunlight and reduce the amount of heat trapped by greenhouse gases.

Long and his colleagues don’t yet have enough data to know how much of an effect the icy haze left by airplanes may be having on the climate or whether it is contributing to warming or cooling. But its existence demonstrates yet another way that humans might be altering the climate system, Long says, and “you can see this with your own eyes.”

The discovery comes out of studies of how much sunlight reaches the Earth’s surface. This energy is not constant. From the 1950s to the 1980s, for instance, the sun seemed to slightly dim, then it started to brighten.

When scientists looked for a cause, they tried linking these changes to the sun’s variable output, said Martin Wild of the Institute for Atmospheric and Climate Science at ETH Zurich during the press conference. But they couldn’t find any correlations.

“If it’s not the sun, it must be the atmosphere” responsible for the change, he said. High levels of pollution in the mid-20th century sent massive amounts of aerosols into the atmosphere, where they blocked some of the sun’s energy. But when places like the United States and Europe began polluting less, the amount of aerosols decreased, and the sun appeared to slightly brighten.

Long and his colleagues found intriguing data that something else is also going on. Some of the sun’s light travels directly to the Earth’s surface, but some of it gets scattered during the trip through the atmosphere. With less pollution, this diffuse light should have decreased, but instead it appeared to be increasing.

“We’ve got a mystery here,” Long said. There must be something in the atmosphere scattering the sun’s light. “Small ice particles fit the bill,” he said.

Long thinks air traffic is the most likely source of those particles. Exhaust from an airplane engine contains aerosols and water vapor. High in the atmosphere, where it is extremely cold, the particles serve as nuclei for ice crystals, which form the bright contrails seen in a plane’s wake. Some of these contrails, scientists have found, can contribute to climate change.

As a contrail dissipates, it leaves behind a thin, icy haze. The sky may appear cloud free, but the particles are there until they fall out of the atmosphere. And while in the sky, they scatter the sun’s light in a similar way as in the proposed geoengineering projects.

“It seems quite possible that [Long is] seeing something that’s real,” says Kevin Trenberth of the National Center for Atmospheric Research. But there’s a lot more work that needs to be done to verify the find and make a connection to the climate.

Skies aren’t clear all the time, and why they are clear one day and not the next could matter. “The reason for the clear sky is a factor [Long] needs to explore more,” Trenbeth says.

Long admits that his study creates far more questions than it answers. So far he has good data for only one spot in Oklahoma, and there could be less sky whitening in places that don’t get as much air traffic. The answers to these questions could help scientists better determine what effect airplane travel is having on the global atmosphere and improve climate models, so we can better understand what will happen on our planet in the future.